1. Area of the Invention
This invention relates to phase and frequency locked loops and more particularly relates to phase and frequency locked loops having multiple inputs.
2. Description of the Prior Art
In many communication applications, each node in the network has its own internal clock running independently of the other clocks in the network. Where the networks are synchronous networks such as in many telecommunications applications and in high speed wide area networks, however, these clocks must be syntonized or synchronized to each other. In certain networks where the concentration of traffic in very large central offices permits expensive clocking solutions such as American Telephone and Telegraph's long distance network, multiple primary reference clocks (PRC's) are distributed throughout the network. Each of these PRC's ensembles multiple global positioning satellite signals received by GPS receivers and steers several rubidium oscillators to track the ensembled GPS time, which serves to represent a universal time scale throughout the network. Since the cost of a PRC is quite high, for other nodes throughout the network, the local oscillator clock for a given node is synchronized or syntonized to the PRC clock through a locked loop. The traceability of the local clock back to universal time depends upon the overall network architecture and the use of expensive oscillators and minimizing the number of cascaded clocks from the PRC site to the local office. The cost of all of the supporting processing elements throughout the network is quite high.
For networks having a relatively small number of nodes (on the order of several hundred) and where there is a high volume of traffic, such costs for maintaining a high degree of traceability may be justifiable. It is more difficult to bear these costs for lower traffic networks such as privately operated nodes coupled to long distance switching networks. Also, in networks having many more nodes such as in digital cellular telephone networks, data networks such as ARDIS, at least some paging systems and PCS, the nodes should preferably be syntonized to each other. Given either the low amount of traffic, or the immense number of nodes, it is expensive and often impractical to have a large number of nodes having clocks traceable to universal time using conventional techniques scattered throughout the network.
As alternatives, there are several alternative sources for timing signals. One example is GPS; however, the short term stability and reliability make it unusable as a direct source of timing in network applications. While the long term stability of a signal such as GPS is generally very good, even exceeding the stability of atomic clocks, the short term stability of GPS signals makes it totally unreliable. In particular, the short term timing solution recovered from a GPS receiver is impaired by both linear and non-linear noise components introduced by the source, communication channel and the receiver. The result is that the recovered timing signal short term stability is characterized by both short term noise such as white noise phase modulation and white noise frequency modulation and by short outages and phase transients. The short term instability and reliability of such received GPS signals makes them inadequate for timing a variety of nodes in a network.
To reduce cost, lower cost quartz based oscillators can be used to replace Rubidium as the local flywheel oscillator in a GPS receiver base PRC. Oven base quartz oscillators still are relatively costly (several hundred dollars or more) and produce significant timing instability over the flywheel times required to manage and suppress the short term noise on the GPS timing signal. Other even lower cost non-oven based oscillator solutions are completely unsuitable because their medium term stability is even worse. Another fundamental problem with using a GPS receiver with a single local oscillator is if there is excessive instability in the control loop, there is no independent means to determine if the source is the GPS receiver or the local oscillator.
As an alternative clock signal source, timing information can be extracted from telephony network based reference signals such as received OC-3 or DS-1 signals. However, these signals often have a worse frequency stability than a quartz oscillator for short time measurement periods on the order of seconds while the OC-3 or DS- 1 signal has a worse frequency stability than GPS over longer measurement periods on the order of tens of minutes. Further, the stability of the timing signals extracted from E1 or DS-1 signals over the medium measurement periods from several seconds to tens of minutes is normally better than GPS or a local quartz oscillator. As a result, there is no one signal that can be used as a timing source for frequency stability throughout short, intermediate and long term stability constraints that is economically available.
Another alternative is to use an ensemble of local oscillators to improve the stability of the overall quartz oscillator resource and provide fault detection. However this requires at least two local oscillators at each location which is a significant cost issue.
Therefore, it is a first object of this invention to provide good stability clock source for the short term, intermediary and long term measurement intervals. It is yet another object of this invention to provide such a timing source that is reproducible throughout an entire network having a large number of nodes without a substantial per node cost.